The present invention relates to a method for producing a filter including a microporous membrane having a supporting structure connected thereto, as well as to a filter produced according to this method.
Federal Republic of Germany DE-OS No. 3,332,345, corresponding to U.S. Pat. No. 4,655,797, discloses a fine screen in the form of a coated metal fabric in which the coating is formed by a galvanic or chemical metal deposition which reduces the width of the screen mesh. In such fine screens, the retention capability or filter gauge, respectively, can be controlled by monitoring the deposition and, without difficulty to the mesh down into regions around 0.01 .mu.m, to set the openings at a defined width. However, with the reduction in mesh width, the transparency or porosity of the fine screen drops as well.
Federal Republic of Germany DE-OS No. 2,440,081, corresponding to U.S. Pat. No. 4,356,215 discloses a filter which includes a macroporous carrier whose pore diameter is about 15 to 20 microns. This carrier is successively coated with two coatings, with the second coating having a pore diameter of about 1 to 2 .mu.. The thus treated carrier may then be connected with a microporous layer. A similarly configured filter is disclosed in DE-OS No. 3,546,328. In the filter of this reference, the substrate is to have an average pore size between about 0.5 .mu.m and about 10 .mu.m, while the average pore size of the microporous membrane is to be about 0.1 to 1 .mu.m. In the examples, the substrate had a pore size of 0.9 .mu.m and the microporous membrane had a pore size of about 0.08 to 0.3 .mu.m. However, DE-OS No. 2,440,081, and corresponding U.S. Pat. No. 4,356,215, already mention that an attempt must be made to increase the diameter of the pores of the carriers without enlarging the diameter of the pores of the microporous layer in order to improve the quality of the carrier, i.e. increase its permeability. On the other hand, the pore sizes in these filters are subject to great statistical fluctuations (see page 10, lines 3 et seq. of DE-OS No. 3,546,328), which makes it more difficult to stay within close tolerances.